Composite filter and method of making the same

ABSTRACT

The invention refers to a composite filter for filtering a stream of ambient air including at least one non-prebonded upstream tier and one non-prebonded downstream tier, wherein the ratio of absolute pore volume of upstream tier to downstream tier RAPV&gt;2, and the absolute projected fiber coverage of upstream tier and of downstream tier APFC&gt;95%. Further, the invention refers to a method of making such a composite filter including the steps of (a) laying down a filtration material onto a support to form the upstream non-prebonded tier, (b) depositing onto the upstream tier the downstream non-prebonded tier, and (c) bonding the tiers to form a composite filter having a unitary stratified structure.

This application is based on, and claims priority to InternationalApplication No. PCT/EP02/02251, filed Mar. 1, 2002 and EuropeanApplication No. 011 05 152.1, filed Mar. 2, 2001.

FIELD OF THE INVENTION

The invention relates, to a composite filter for removing solidparticles entrained in a stream of ambient air. More specifically, itrelates to a composite filter comprising at least one non-prebondedupstream tier and one non-prebonded downstream tier useful for filteringparticulates from ambient air.

The term “pre-bonded” means herein that a composition of filter medium,such as thermally bondable fusing fibers or adhesively bindable fibers,is treated in a manner effective to activate the binding mechanismthereby forming a separate, free-standing, cohesive, and typicallyself-supporting web of that filter composition. Such a pre-bonded webcan be mechanically manipulated by such processes as winding on a roll,unwinding from a roll, cutting and the like.

The term “tier” herein means a band formed from non-prebonded filtermaterial into a stratum of unitary stratified structure. In contrast, a“layer” means a separately, prebonded, self-supporting web of filtermaterial.

BACKGROUND AND PRIOR ART

In recent times, the technology for filtering particulates from gaseshas become quite sophisticated in both commonplace applications such asconsumer oriented vacuum cleaning of dirt and dust as well as verydemanding industrial applications such as removal from gases of specificparticle size fractions of wide varieties of contaminants including frominert to biochemically sensitive, among others. It is now wellappreciated that the contaminating particulates in a gas stream can havea wide variety of sizes, geometric shapes, e.g., elongated andspherical, and chemical and physical compositions, e.g., odor-free andodoremitting particles.

Consequently, filtration technology has evolved to provide filter mediawhich are adapted to optimally filter specific fractions of thecontaminating particulates. Also, this technology has developedtechniques for optimizing various performance characteristics of filterssuch as maintaining low pressure drop across the filter and increasingthe filter service life so as to extend the length of time betweenfilter element replacements.

The traditional approach to achieving these objectives has been toprovide a multilayer filter medium composed of separate, individuallydesigned layers which are each intended to accomplish primarily one, andsometimes several specific filter functions. For example, a very open,porous and thin scrim is often used to protect underlying filter layersfrom abrasion by fast moving, large and hard particles; a porous andbulky layer is typically used to capture substantial amounts of chieflylarge particles, and an ultrafine diameter filament, low porosity layeris usually prescribed for removing the smallest particles to increasefiltration efficiency. From the many choices available, separate filterlayers are selected and combined in a preselected sequence thenassembled as a group to form a multilayer, and therefore multifunctionalfilter. The one or more adjacent layers can be bonded to each other orthe layers can be unbonded. Optionally, the individual layers can besandwiched between covers, typically of paper, for structural integrityand ease of handling.

A drawback of the aforementioned multilayer system of constructingmultifunctional filters is that there is repetitive processing of thefilter media which can be excessive. That is, the filter material in agiven layer is first processed to form the individual layer, then it isprocessed to assemble that layer in the multilayer filter. Each stepadds to the compaction and cover, if ever slight, of the final filterproduct. This tends to raise the pressure drop through the filter andreduce dust holding capacity, thereby limiting service life.

WO 01/03802 discloses a composite filter comprising at least onenon-prebonded upstream tier and one non-prebonded downstream tier.However, as will be shown in detail below (FIG. 2), in this compositefilter a relatively high pressure drop across the composite filteroccurs. Further, also the service life time of this filter is low.

In view of this, the objective problem underlying the invention is toprovide a composite filter in which the pressure drop across the filteris maintained low and which has a long service life time.

SUMMARY OF THE INVENTION

This objective problem is solved by a composite filter for filtering astream of ambient air comprising at least one non-prebonded, upstreamtier and one non-prebonded downstream tier, wherein the ratio ofabsolute pore volume of upstream tier to downstream tier RAPV>2, and theabsolute projected fiber coverage of upstream tier and of downstreamtier APFC>95%.

Due to the parameters of this composite filter, the pressure drop acrossthe filter medium is kept low and the service life time of the filter isincreased.

Further, this invention enables to provide a composite filter made up ofat least two stacked tiers of filtration material bonded together toform a unitary stratified structure. The composition of filtrationmaterial in any given tier is preselected to perform a desired filteringfunction. For example, fine, (i.e., small diameter) and densely packedfibers can be selected to capture very small dust particles such asthose of about 5 micrometers and smaller. Additionally,electrostatically charged fibers can also be used to stop passage ofthese and even smaller particles. Similarly, bulky, highly porous mediadesigned to have large dust holding capacity can be utilized to trapmedium to large size dirt particles.

Since the composite filter of the invention comprises non-pre-bondedtiers, the bonding of at least one and preferably all of the tiers toform the unitary structure is begun only after the stacking of all thetiers of a particular desired composite filter structure has beencompleted. The resulting structure is a single body composed ofdifferent types of filtration material which appear as distinct strata.

In view of this, the stratified structure is formed by building up astack of tiers of selected filtration materials. Because the tiers arenon-prebonded, the components of each tier, that is, fibers, granules,etc., generally are laid loosely by mechanical or air-laying processesonto the layer lying below. Within each tier the composition of filtermaterial is largely uniform and there is a “fuzzy” interface between thetiers.

Preferably, the composite filter of the above mentioned kind comprises aratio of average pore diameter of upstream to downstream tier RPD in therange of 4<RPD<10.

Due to this ratio, the dust holding capacity of the upstream tier isgreatly increased, such that the upstream tier acts as a pre filter forthe downstream tier without increasing the pressure drop across thecomposite filter.

Additionally but not exclusively, such composite filter may comprise anaverage pore diameter of the upstream tier PDU, with PDU>60 μm,preferably in the range 80 μm<PDU<200 μm.

All above discussed composite filters may comprise upstream tiers with arelative pore volume RPVU>94%, preferably RPVU>96%, an apparent densityADU<0.05 g/cm³, and a thickness D in the range of 0.5 mm<D<2.5 mm.Choosing these parameters results in an upstream tier with the requiredRAPV and APFC.

Further, this composite filters may also comprise downstream tiers witha relative pore volume RPVD being smaller than RPVU, an apparent densityADD in the range of 0.07 g/cm³<ADD<0.14 g/cm³, and a thickness D in therange of 0.1 mm<D<0.4 mm. Choosing these parameters results in andownstream tier with the required RAPV and APFC.

Further, the upstream tier of any of the above discussed compositefilter may preferably comprise fibers having a length in the range of0.1 mm to 3.0 mm.

Due to such a structure, the upstream tier can be made bulkier toprovide greater dust holding capacity.

Preferably, the above discussed composite filters may comprise anupstream tier having a dust retention DR with respect to dust particleswith a diameter corresponding to the average pore diameter of thedownstream tier of DR>99%.

This feature avoids an clogging of the downstream tier, and therefore,further maintains a low pressure drop across the filter and furtherincreases the service life time of the composite filter.

Additionally but not exclusively, this effect can be increased in acomposite filter in which the orientation of the fibers in flowdirection in the upstream tier is higher than in the downstream tier.Such structure further improves maintaining the pressure drop across thefilter.

The composite filters, as discussed above, may comprise an upstream tierof dry-laid, thermally bondable fusing, bicomponent or monocomponentpolymer fibers and a downstream tier of meltblown fibers. In this aspecta single tier is constituted of a single type of filter medium, forexample, 100% bicomponent polymer fibers, melt blown, staple fibers, orspunbond filaments.

Alternatively, the composite filter may comprise an upstream tier havinga composition selected from the group consisting of 100 wt % bicomponentpolymer fibers, a blend of at least about 10 wt % bicomponent polymerfibers with a complementary amount of natural fibers, such as fluff pulpfibers or kokon fibers, staple fibers or a mixture thereof, and a blendof at least about 10 wt % monocomponent polymer thermally bondablefusing fibers with a complementary amount of fluff pulp fibers, staplefibers or a mixture thereof.

In this aspect, a single tier is constituted by a blend of media, suchas an air-laid, usually uniform blend of bicomponent polymer fibers andfluff pulp (FP) fibers.

Since it is also desirable to provide a stratified structure, adjacenttiers in a stack may have different compositions. Nonetheless, acomposition of one tier can be repeated in a stack although at least onetier of different composition should be present between the tiers ofsame composition.

This structure of the composite filter differs from that of conventionalmultilayer filtration media which are formed by laminating a pluralityof individual filter medium layers that have each been pre-bonded toform a self-supporting web prior to formation of the multilayerlaminate.

Such unitary stratified structure provides a number of significantadvantages over conventional filter media. In one aspect, the unitarystratified structure can be made bulkier to provide greater dust holdingcapacity than a laminate of individually, pre-bonded layers havingcompositions corresponding respectively to the tiers of the unitarystructure. This is because each portion of the conventional filtermedium is compressed at least twice: once when the individual layer isformed by bonding, and a second time when the individual layers arelaminated to form the filter.

Preferably, the bicomponent polymer fibers of this structure may have asheath of one polymer and a core of a different polymer having a meltingpoint higher than the one polymer. The core may comprise polypropyleneand the sheath may comprise polyethylene.

Additionally, the core may be disposed eccentric relative to the sheath.In such a structure, the fibers will crimp with the result that thebulkiness of the tier is further increased.

Preferably and alternatively, the above discussed composite filter maycomprise an upstream tier further having fibers selected from at leastone of uncharged split film fibers, charged split film fibers and mixedelectrostatic fibers.

Accordingly, the present invention now provides a composite filtercomprising at least two nonprebonded tiers each tier independentlycomprising at least one filtration material and being distinct from theadjacent tier, in which the tiers are bonded together to form a unitarystratified structure having a first boundary surface adapted to receiveparticulates entrained in air and a second boundary surface adapted todischarge filtered air, this composite filter showing a reduced pressuredrop and a prolonged service life time.

All above discussed composite filters may be embodied in a vacuumcleaner bags, and more generally in vacuum filters. By “vacuum filter”is meant a filter structure intended to operate by passing a gas,preferably air, which entrains usually dry solid particles, through thestructure. The convention has been adopted in this application to referto the sides, tiers and layers of the structure in relation to thedirection of air flow. That is, the filter inlet side is “upstream” andthe filter discharge side is “downstream” for example. Occasionallyherein the terms “in front of” and “behind” have been used to denoterelative positions of structure elements as being upstream anddownstream respectively. Of course, there will be a pressure gradient,sometimes referred to as “pressure drop”, across the filter duringfiltration. Vacuum cleaners typically use bag shaped filters. Normally,the upstream side of a vacuum bag filter is the inside and thedownstream side is outside.

In addition to vacuum cleaner bags, the composite filter of theinvention can be utilized in applications such as heating ventilationand air conditioning (HVAC systems, vehicle cabin air filters, highefficiency (so-called “HEPA”) and clean room filters, emission controlbag house filters, respirators, surgical face masks and the like.Optionally, the composite filter can be used in such applications withan additional carbon fiber or particle-containing layer in series withthe composite filter of the invention, for example to absorb odors ortoxic contaminants. Moreover, certain applications, such as HEPA andclean room filters can employ additional layers in series with thecomposite filter of the invention, such as low porositypolytetrafluorethylene (PTFE) membrane laminated to a boundary surfaceof an appropriate unitary stratified structure, composite filter.

The present invention also provides a method of making a compositefilter of the above kind, comprising the steps of

-   -   (a) laying down a filtration material onto a support to form the        upstream non-prebonded tier,    -   (b) depositing onto the upstream tier the downstream        non-prebonded tier, and    -   (c) bonding the tiers to form a composite filter having a        unitary stratified structure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram showing in cross section an embodiment ofthe composite filter in accordance with the invention having a unitarystratified structure of two tiers.

FIG. 2 is a diagram showing the pressure drop of the composite filter inFIG. 1 and a composite prior art filter.

FIG. 3 is a schematic diagram showing in cross section anotherembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of three tiers.

FIG. 4 is a schematic diagram showing in cross section anotherembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of four tiers.

FIG. 5 is a schematic diagram showing in cross section anotherembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of five tiers.

FIG. 6 is a schematic diagram showing in cross section anotherembodiment of the two-tiered composite filter of FIG. 1 in combinationwith a filter layer adjacent thereto.

FIG. 7 is a schematic diagram showing in cross section anotherembodiment of the three-tiered composite filter of FIG. 3 in combinationwith a filter layer adjacent thereto.

FIG. 8 is a schematic diagram showing in cross section anotherembodiment of the four-tiered composite filter of FIG. 4 in combinationwith a filter layer adjacent thereto.

FIG. 9 is a schematic diagram showing in cross section anotherembodiment of the five-tiered composite filter of FIG. 5 in combinationwith a filter layer adjacent thereto.

FIG. 10 is a schematic cross section diagram showing the two-tieredcomposite filter of FIG. 6 bonded to an adjacent filter layer with anadhesive or ultrasonically bonded layer.

FIG. 11 is a schematic cross section diagram showing the three-tieredcomposite filter of FIG. 7 bonded to an adjacent filter layer with anadhesive or ultrasonically bonded layer.

FIG. 12 is a schematic cross section diagram showing the four-tieredcomposite filter of FIG. 8 bonded to an adjacent filter layer with anadhesive or ultrasonically bonded layer.

FIG. 13 is a schematic cross section diagram showing the five-tieredcomposite filter of FIG. 9 bonded to an adjacent filter layer with anadhesive or ultrasonically bonded layer.

FIG. 14 is a schematic diagram of an inline process for producing acomposite filter according to a preferred embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, and before discussing explicitly discussing thepreferred embodiments of the invention, different filter materialcompositions which suitably used in the present invention are describedin greater detail:

Regarding the discussion below, DIN 44956-2 test has been employed todetermine the increase in pressure drop of five different examples ofvacuum cleaner bag constructions after dust loading with fine dust atthe following levels: 0, 0.5, 1.0, 1.5, 2.0, and 2.5 grams.

Air Permeability after Fine Dust Loading Test: The dust loading part ofthe DIN 44956-2 is performed at 0.5 gram increments from 0 to 2.5g/(m²×s) on seven bags of each sample. However, the pressure drop valuesare not recorded again. The maximum sustainable air permeability valuesare then determined on the bags, which had the specified levels of dustloading.

Standard Vacuum Cleaner Filter Bag Material:

This material, sometimes referred to as “standard paper” hastraditionally been used as a single ply in which it provides dustfiltration and containment, as well as the strength and abrasionresistance required of a vacuum cleaner bag. This material is also rigidenough to enable easy fabrication on standard bag manufacturingequipment. The paper is predominantly composed of unbleached wood pulpwith 6–7% of a synthetic fiber such as poly[ethylene terephthalate](PET) type polyester, and is produced by the wet laying process. Thestandard paper typically has a basis weight of about 30–80 g/m² andcommonly about 50 g/m². The PET fibers typically have a fineness of 1.7dtex and lengths of 6–10 mm. This paper has air permeability in therange of about 200–500 L/(m²×s) and an average pore size of about 30 mm.However, the efficiency as determined from the DIN 44956-2 Test is onlyabout 86%. Another characteristic is that the pores are quickly cloggedwith dust and the dust holding capacity is further limited-by the verythin paper thickness of only about 0.20 mm.

Spunbond Nonwoven:

A nonwoven of spunbond polymer fibers can be deployed as a filtrationtier in the structure. The fibers can be of any spunbond-capable polymersuch as polyamides, polyesters or polyolefins. Basis weight of thespunbond nonwoven should be about 10–100 g/m² and preferably about 30–40g/m². The spunbond nonwoven should have an air permeability of about500–10,000 L/(m²×s), and preferably about 2,000–6,000 L/(m²×s) asmeasured by DIN 53887. The spunbond can also be electrostaticallycharged.

Scrim or Supporting Fleece:

Scrim refers to a generally light basis weight, very open porous paperor nonwoven web. Basis weight of the scrim is typically about 10–30g/m², and frequently about 13–17 g/m². The scrim, sometimes referred toas a supporting fleece usually has air permeability of about 500–10,000L/(m²×s). It is primarily employed to protect other tiers or layers fromabrasion. The scrim can also filter the very largest particles. Thescrim, as well as any tier of the filter composite, can beelectrostatically charged provided the material has suitable dielectricproperties.

Wet-Laid High Dust Capacity Material:

Wet-laid High Dust Capacity material, frequently referred to herein as“wet-laid capacity paper” is bulkier, thicker and more permeable thanthe standard vacuum cleaner bag filter paper. It performs multiplefunctions. These include resisting shock loading, filtering of largedirt particles, filtering a significant portion of small dust particles,holding large amounts of particles while allowing air to flow througheasily, thereby providing a low pressure drop at high particle loadingwhich extends the life of the filter.

The wet-laid capacity paper usually comprises a fiber mixture of woodpulp fibers and synthetic fibers. It typically contains up to about 70%wood pulp and correspondingly more synthetic fiber, such as PET, thanthe standard paper described above. It has a greater thickness than thestandard paper of about 0.32 mm at a typical basis weight of 50 g/m².Pore size also is much greater, in that the average pore size can begreater than 160 mm. Thus, the paper is able to hold much more dust inits pores before clogging up. Basis weight of the wet-laid capacitypaper typically is about 30–150 g/m² and preferably about 50–80 g/m².

The wet-laid capacity paper has a fine dust particle filtrationefficiency of about 66–67% as determined by the DIN 44956-2.Importantly, the wet-laid capacity paper has air permeability higherthan the standard filter paper. The permeability lower limit thuspreferably should be at least about 500 L/(m²×s), more preferably atleast about 1,000 L/(m²×s) and most preferably at least about 2,000L/(m²×s). The upper limit of permeability is defined to assure that thepaper filters and holds a major fraction of the dust particles largerthan about 10 mm. Consequently, a secondary high efficiency filtermedium positioned downstream is able to filter out and contain fineparticles much longer before showing indication of a substantialpressure drop increase across the filter. Accordingly, the airpermeability of the wet-laid capacity paper preferably should be at mostabout 8,000 L/(m²×s), more preferably at most about 5,000 L/(m²×s), andmost preferably at most about 4,000 L/(m²×s). It is thus seen that thewet-laid capacity paper is especially well designed as a multipurposefiltration tier to be positioned upstream of the secondary highefficiency filtration tier.

Dry-Laid High Dust Capacity Material:

Dry-laid high dust capacity material, sometimes referred to herein as“dry-laid capacity paper”, had not been used as a filter in vacuumcleaner bags. Dry-laid paper is not formed from a water slurry, but isproduced with air-laying technology and preferably by a “fluff pulp”process. Hydrogen-bonding which plays a large roll in attracting themolecular chains together does not operate in the absence of water.Thus, at the same basis weight, dry-laid capacity paper, is usually muchthicker than standard paper and the wet-laid capacity paper. For atypical weight of 70 g/m², the thickness is 0.90 mm, for example.

The dry-laid capacity paper webs can be bonded primarily by two methods.The first method is latex bonding in which the latex binder may beapplied from water-based dispersions. Saturation techniques such asspraying or dipping and squeezing (padder roll application) followed inboth cases by a drying and beat curing process can be used. The latexbinder may also be applied in discrete patterns such-as dots diamonds,cross hatches or wavy lines by gravure roll followed by drying andcuring.

The second method is thermal bonding, for example by utilizing binderfibers. Binder fibers sometimes referred to herein as “thermallybondable fusing fibers” are defined by the Nonwoven Fabric Handbook,(1992 edition) as “Fibers with lower softening points than other fibersin the web. Upon the application of heat and pressure, these act as anadhesive.” These thermally bondable fusing fibers generally completelymelt at locations where sufficient heat and pressure are applied for theweb, thereby adhering the matrix fibers together at their cross-overpoints. Examples include co-polyester polymers which when heated adherea wide range of fibrous materials.

In a preferred embodiment thermal bonding can be accomplished by addingfrom at least 20% preferably up to 50% of a bicomponent (“B/C”) polymerfiber to the dry-laid web. Examples of B/C fibers include fibers with acore of polypropylene (“PP”) and a sheath of more heat sensitivepolyethylene (“PE”). The term “heat sensitive” means that thermoplasticfibers soften and become sticky or heat fusible at a temperature of 3–5degrees C. below the melting point. The sheath polymer preferably shouldhave a melting point in the range of about 90–160 degrees C. and thecore polymer should have a higher melting point, preferably by at leastabout 5 degrees C. higher than that of the sheath polymer. For example,PE melts at 121 degrees C. and PP melts at 161–163 degrees C. This aidsin bonding the dry-laid web when it passes between the nip of a thermalcalendar or into a through-air oven by achieving thermally bonded fiberswith less beat and pressure to produce a less compacted, more open andbreathable structure. In a more preferred embodiment the core of thecore/sheath of the B/C fiber is located eccentric of the sheath. Themore that the core is located towards one side of the fiber the morelikely that the B/C fiber will crimp during the thermal bonding step,and thereby increase the bulk of the dry-laid capacity. This will, ofcourse, improve its dust holding capacity. Thus, in a still furtherpreferred embodiment the core and sheath are located side-by-side in theB/C fiber, and bonding is achieved with a through-air oven. A thermalcalendar, which would compress the web more than through-air bonding andis less preferred in this case. Other polymer combinations that may beused in core/sheath or side-by-side B/C fibers include PP withco-polyester, low melting polymers, and polyester with nylon 6. Thedry-laid high capacity tier can also be constituted essentiallycompletely by bicomponent fibers. Other variations of bicomponent fibersin addition to “sheath/core”, can be used, such as “side-by-side”,“islands in the sea” and “orange” embodiments disclosed in NonwovenTextiles, Jirsak, O., and Wadsworth, L. C., Carolina Academic Press,Durham, N.C., 1999, pp. 26–29.

Generally, the average pore size of dry-laid capacity is intermediatebetween the pore size of the standard paper and wet-laid capacity paperThe filtration efficiency as determined by the DIN 44956-2 Test isapproximately 80%. Dry-laid capacity paper should have about the samebasis weight and the same permeability as the wet-laid capacity paperdescribed above, i.e., in the range of about 500–8,000 L/(m²×s),preferably about 1,000–5,000 L/(m²×s) and most preferably about2,000–4,000 L/(m²×s). It has excellent dust holding capacity and has theadvantage of being much more uniform in weight and thickness than thewet-laid papers.

Several preferred embodiments of dry-laid capacity paper arecontemplated. One is a latex bonded fluff pulp fiber composition. Thatis, the fibers comprising the paper consist essentially of fluff pulp.The term “fluff pulp” means a nonwoven component of the filter of thisinvention which is prepared by mechanically grinding rolls of pulp,i.e., fibrous cellulose material of wood or cotton, then aerodynamicallytransporting the pulp to web forming components of air laying or dryforming machines. A Wiley Mill can be used to grind the pulp. So-calledDan Web or M and J machines are used for dry forming. A fluff pulpcomponent and the dry-laid tiers of fluff pulp are isotropic and arethus characterized by random fiber orientation in the direction of allthree orthogonal dimensions. That is, they have a large portion offibers oriented away from the plane of the nonwoven web, andparticularly perpendicular to the plane, as compared tothree-dimensionally anisotropic nonwoven webs. Fibers of fluff pulputilized in this invention preferably are from about 0.5–5 mm long. Thefibers are held together by a latex binder. The binder can be appliedeither as powder or emulsion.

Binder is usually present in the dry-laid capacity paper in the range ofabout 10–30 wt % and preferably about 20–30 wt % binder solids based onweight of fibers.

Another preferred embodiment the dry-laid capacity paper comprises athermally bonded blend of fluff pulp fibers and at least one of “splitfilm fibers” and bicomponent polymer fibers. More preferably, the blendof fluff pulp fibers comprises fluff pulp fibers and bicomponent polymerfibers.

Split Film Fibers:

Split film fibers are essentially flat, rectangular fibers which may beelectrostatically charged before or after being incorporated into thecomposite structure of the invention. The thickness of the split filmfibers may range from 2–100 micrometers, the width may range from 5micrometers to 500 micrometers, and the length may range from 0.5 to 15mm. However, the preferred dimensions of the split film fibers are athickness of about 5 to 20 micrometers, a width of about 15 to 60micrometers, and a length of about 0.5 to 8 mm.

The split film fibers of the invention are preferably made of apolyolefin, such as polypropylene. However, any polymer which issuitable for making fibers may be used for the split film fibers of thecomposite structures of the invention. Examples of suitable polymersinclude, but are not limited to, polyolefins like homopolymers andcopolymers of polyethylene, polyterephthalates, such as poly(ethyleneterephthalate) (PET), poly(butylene terephthalate) (PBT),poly(cyclohexyl-dimethylene terephthalate) (PCT), polycarbonate, andpolychlorotrifluoroethylene (PCTFE). Other suitable polymers includenylons, polyamides, polystyrenes, poly-4-methylpentene-1,polymethylmethacrylates, polyurethanes, silicones, polyphenylenesulfides. The split film fibers may also comprise a mixture ofhomopolymers or copolymers. In the present application, the invention isexemplified with split film fibers made of polypropylene.

The use of PP polymers with various molecular weights and morphologiesin laminate film structures has been shown to produce films with aproper balance of mechanical properties and brittleness required toproduce split film fibers. These PP split film fibers may also besubsequently given the desired level of crimp. All dimensions of thesplit film fibers may, of course, be varied during manufacture of thefibers.

One method for production of the split fibers, is disclosed in U.S. Pat.No. 4,178,157. Polypropylene is melted and extruded into a film which isthen blown into a large tube (balloon) into which ambient air isintroduced or allowed to enter, in accordance with conventional blowstretching technology. Inflating the balloon with air serves to quenchthe film and to bi-axially orient the molecular structure of the PPmolecular chains, resulting in greater strength. The balloon is thencollapsed and the film is stretched between two or more pairs of rollersin which the film is held in the nip of two contacting rollers, with theapplication of varying amounts of pressure between the two contactingrollers. This results in an additional stretch in the machine directionwhich is accomplished by driving the second set of rollers at a fastersurface speed than the first set. The result is an even greatermolecular orientation to the film in the machine direction which willsubsequently become the long dimension of the split film fibers.

The film may be electrostatically charged before or after it has beencooled down. Although various electrostatic charging techniques may beemployed to charge the film, two methods have been found to be mostpreferable. The first method involves passing the film about midway in agap of about 1.5 to 3 inches between two DC corona electrodes. Coronabars with emitter pins of metallic wire may be used in which one coronaelectrode has a positive DC voltage potential of about 20 to 30 kV andthe opposing electrode has a negative DC voltage of about 20 to 30 kV.

The second, preferred, method utilizes the electrostatic chargingtechnologies described in U.S. Pat. No. 5,401,446 (Wadsworth and Tsai,1995), which is referred to as Tantret(tm) Technique I and Technique II,which are further described herein. It has been found that Technique II,in which the film is suspended on insulated rollers as the film passesaround the inside circumference of two negatively charged metal shellswith a positive corona wire of each shell, imparts the highest voltagepotentials to the films. Generally, with Technique II, positive 1,000 to3,000 volts or more may be imparted to on one side of the films withsimilar magnitudes of negative volts on the other side of the chargedfilm. Technique I, wherein films contact a metal roller with a DCvoltage of −1- to −10 kV and a wire having a DC voltage of +20 to +40 kVis placed from about 1 to 2 inches above the negatively biased rollerwith each side of the film being exposed in succession to thisroller/wire charging configuration, results in lower voltage potentialsas measured on the surfaces of the films. With Technique I, voltages of300 to 1,500 volts on the film surface with generally equal but oppositepolarities on each side are typically obtained. The higher surfacepotentials obtained by Technique II, however, have not been found toresult in better measurable filtration efficiencies of the webs madefrom the split film fibers. Therefore, and because it is easier to laceup and pass the film through the Technique I device, this method is nowpredominately used to charge the films prior to the splitting process.

The cooled and stretched film may be hot or cold electrostaticallycharged. The film is then simultaneously stretched and split to narrowwidths, typically up to about 50 micrometers. The split, flat filamentsare then gathered into a tow which is crimped in a controlled numbers ofcrimps per centimeter and then cut into the desired staple length.

In a particularly preferred embodiment, the dry-laid high dust capacitypaper comprises a blend of all of fluff pulp fibers, bicomponent polymerfibers, and electrostatically charged split film fibers. Preferably, thefluff pulp fibers will be present at about 5–85 wt %, more preferablyabout 10–70 wt %, and most preferably about 40 wt %, the bicomponentfibers at about 10–60 wt %, more preferably about 10–30 wt % and mostpreferably about 20 wt %, and the electrostatically charged split filmfibers at about 20–80 wt %, and more preferably about 40 wt %. Thisdry-laid high dust capacity may be thermally bonded, preferably at ahigh temperature of 90–160 degrees C., more preferably, at a temperaturelower than 110 degrees C. and most preferably at about 90 degrees C.

Mixed Electrostatic Fibers:

Other preferred embodiments of the dry-laid capacity paper comprises athermally bonded paper with 100% “mixed electrostatic fibers”, a blendof 20–80% mixed electrostatic fibers and 20–80% B/C fibers, and a blendof 20–80% mixed electrostatic fibers, 110–70% fluff pulp and 10–70% B/Cfibers. “Mixed electrostatic fiber” filters are made by blending fiberswith widely different triboelectric properties and rubbing them againsteach other or against the metal parts of machines, such as wires oncarding cylinders during carding. This makes one of the types of fibersmore positively or negatively charged with respect to the other type offiber, and enhances the coulombic attraction for dust particles. Theproduction of filters with these types of mixed electrostatic fibers istaught in U.S. Pat. No. 5,470,485 and in European Patent Application EP0 246 811.

In U.S. Pat. No. 5,470,485, the filter material consists of a blend of(I) polyolefin fibers and (II) polyacrylonitrile fibers. The fibers (I)are bicomponent PP/PE fibers of the core/sheath or side-by-side type.The fibers 11 are “halogen free”. The (I) fibers also have some“halogen-substituted polyolefins”: whereas, the acrylonitrile fibershave no halogen. The patent notes that the fibers must be thoroughlywashed with nonionic detergent, with alkali, or solvent and then wellrinsed before being mixed together so that they do not have anylubricants or antistatic agents. Although the patent teaches that thefiber mat produced should be needle-punched, these fibers could also becut to lengths of 5–20 mm and mixed with similar length bicomponentthermal binder fibers and also with the possible addition of fluff pulpso that dry-laid thermally bonded paper can be utilized in thisinvention.

EP 0 246 811 describes the triboelectric effect of rubbing two differenttypes of fibers together. It teaches using similar types of fibers asU.S. Pat. No. 5,470,485, except that the —CN groups of thepolyacrylonitrile fibers may be substituted by halogen (preferablyfluorine or chlorine). After a sufficient amount of substitution of —CNby —Cl groups, the fiber may be referred to as a “modacrylic” if thecopolymer comprises from 35 to 85% weight percent acrylonitrile units.EP 0 246 811 teaches that the ratio of polyolefin to substitutedacrylonitrile (preferably modacrylic) may range from 30:70 to 80:20 bysurface area, and more preferably from 40:60 to 70:30. Similarly, U.S.Pat. No. 5,470,485 teaches that the ratio of polyolefin topolyacrylonitrile fibers is in the range of 30:70 to 80:20, relative toa surface of the filter material. Thus, these ranges of ratios ofpolyolefin to acrylic or modacrylic fibers may be used in the abovestated proportions in the dry-laid thermally bonded capacity paper.

Meltblown Fleece:

A synthetic polymer fiber meltblown fleece can optionally be deployed asan tier between a multipurpose tier and a high efficiency filtrationtier. The meltblown fleece tier increases overall filtration efficiencyby capturing some particles passed by the multipurpose filtration tier.The meltblown fleece tier also optionally can be electrostaticallycharged to assist in filtering fine dust particles. Inclusion of ameltblown fleece tier involves an increase in pressure drop at givendust loading as compared to composites not having a meltblown fleeceter.

The meltblown fleece preferably has a basis weight of about 10–50 g/m²and air permeability of about 100–1,500 L/(m²×s).

High Bulk Meltblown Nonwoven:

Another discovery from recent research to develop improved vacuumcleaner bags was the development of a high bulk MB web or tier whichcould be used upstream of a filtration grade MB fleece as a pre-filterin place of the wet-laid capacity paper or dry-laid capacity paper. Thehigh bulk MB pre-filter can be made in a meltblowing process usingchilled quench air with a temperature of about 10 degrees C. Incontrast, conventional MB normally uses room air at an ambienttemperature of 35–45 degrees C. Also the collecting distance from the MBdie exit to the web take-up conveyer is increased to 400–600 mm in thehigh bulk MB process. The distance normally is about 200 mm for regularMB production. Additionally, high bulk MB nonwoven is made by using alower temperature attenuation air temperature of about 215–235 degreesC. instead of the normal attenuation air temperature of 280–290 degreesC., and a lower MB melt temperature of about 200–225 degrees C. comparedto 260–280 degrees C. for filtration grade MB production. The colderquench air, lower attenuation air temperature, lower melt temperatureand the longer collecting distance cool down the MB filaments more.Removing beat results in less draw down of the filaments, and hence, inlarger fiber diameters than would be found in typical filtration gradeMB webs. The cooler filaments are much less likely to thermally fusetogether when deposited onto the collector. Thus, the High BulkMeltblown nonwoven would have more open area. Even with a basis weightof 120 g/m², the air permeability of the High Bulk Meltblown nonwoven is806 L/(m²×s). By contrast, a much-lighter (e.g., 22 g/m²) filtrationgrade MB PP web had a maximum air permeability of only 450 L/(m²×s). Thefiltration efficiency of the High Bulk MB nonwoven as determined by theDIN 44956-2 Test was 98%. When the two were put together with the HighBulk MB nonwoven on the inside of the bag, the air permeability wasstill 295 L/(m²×s), and the filtration efficiency of the pair was 99.8%.The high bulk meltblown nonwoven can be uncharged, or optionallyelectrostatically charged provided that the nonwoven is of materialhaving suitable dielectric properties.

High Bulk MB nonwoven of this invention should be distinguished from“filtration grade MB” which also is employed in the multitier vacuumfilter structure of this disclosure. Filtration grade MB web is aconventional meltblown nonwoven generally characterized by a low basisweight typically of about 22 g/m², and a small pore size. Additionaltypical characteristics of filtration grade MB nonwoven of polypropyleneare shown in Table 1. A preferred high bulk MB nonwoven of polypropyleneoptimally includes about 5–20 wt % ethylene vinyl acetate. Filtrationgrade MB nonwoven has generally high dust removal efficiency, i.e.,greater than about 99%.

TABLE I More Most Preferred Preferred Preferred Filtration Grade MB PPWeight g/m² 5–100 10–50 25 Thickness, mm 0.10–2 0.10–1 0.26 AirPermeability, L/(m² × s) 100–5,000 100–2,000 450 Tensile Strength, MD, N0.5–15 1.0–10 3.7 Tensile Strength, CD, N 0.5–15 1.0–10 3.2 FiberDiameter, mm 1–15 1–5 2–3 High Bulk MB PP Weight, g/m² 30–180 60–120 80Thickness, min 0.3–3 0.5–2 1.4 Air permeability, L/(m² × s) 300–8,000600–3,000 2,000 Tensile Strength, MD, N 1.0–30 2–20 10 Tensile Strength,CD, N 1.0–30 2–20 9.2 Fiber Diameter, mm 5–20 10–15 10–12

High Bulk MB nonwoven is similar in filter efficiency to dry-laid andwet-laid capacity papers mentioned above. Thus, High Bulk MB nonwoven iswell-adapted to remove large quantities of large dust particles and tohold large amounts of dust. Accordingly, High Bulk MB nonwoven tier issuited for placement upstream of and as a pre-filter for a filtrationgrade MB tier in a vacuum filter structure of this invention.

Spunblown (Modular) Nonwoven:

A new type of meltblowing technology described in Ward, G., NonwovensWorld, Summer 1998, pp. 37–40 is available to produce a Spunblown(Modular) Nonwoven suitable for use as a coarse filter tier in thepresent invention. Optionally, the Spunblown Nonwoven can be utilized asa filtration grade meltblown fleece tier as called for in the novelstructure. Specifications of the Spunblown (Modular) Nonwoven arepresented in Table II.

The process for making the Spunblown (Modular) Nonwoven is generally ameltblown procedure with a more rugged modular die and using colderattenuation air. These conditions produce a coarse meltblown web withhigher strength and air permeability at comparable basis weight ofconventional meltblown webs.

Microdenier Spunbond Nonwoven:

A spunbond (“SB”) nonwoven, occasionally referred to herein asmicrodenier spunbond can also be utilized in this invention in the sameway as the coarse filter tier or the filtration grade meltblown fleecetier previously mentioned. Specifications of microdenier spunbond arelisted in Table II. Microdenier spunbond is particularly characterizedby filaments of less than 12 mm diameter which corresponds to 0.10denier for polypropylene. In comparison, conventional SB webs fordisposables typically have filament diameters which average 20 mm.Microdenier spunbond can be obtained from Reifenhauser GmbH (ReicofilIII), Koby Steel, Ltd., (Kobe-Kodoshi Spunbond Technology) and AsonEngineering, Inc. (Ason Spunbond Technology).

TABLE II More Most Preferred Preferred Preferred Spunblown (Modular)Weight g/m² 10–150 10–50 28 Thickness, mm 0.20–2 0.20–1.5 0.79 Airpermeability, L/(m² × s) 200–4,000 300–3,000 1,200 Tensile Strength, MD,N 10–60 15–40 43 Tensile Strength, CD, N 10–50 12–30 32 Fiber Diameter,micrometer 0.6–20 2–10 2–4 microdenier spunbond PP (Ason, Kobe-Kodoshi,Reicofil III) Weight, g/m² 10–50 20–30 17 Thickness, mm 0.10–0.60.15–0.5 0.25 Air permeability, L/(m² × s) 1,000–10,000 2,000–6,0002,500 Tensile Strength, MD, N 10–100 20–80 50 Tensile Strength, CD, N10–80 10–60 40 Fiber Diameter, micrometer 4–18 6–12 10

Preferred Embodiments

Representative products according to the present invention areillustrated schematically in FIGS. 1, 3–13, and described in greaterdetail as follows. In the figures, air flow direction is indicated byarrow A.

In FIG. 1, a unitary composite filter 36 made from two tiers isdepicted. The upstream (dirty air side) tier 37 is a Dry-Laid FPCapacity tier with the broadest weight of 10–150 g/m², typical weightrange of 20–80 g/m² and with a preferred weight of 75 g/m². The FP layer37 has different blends of pulp fibers and bicomponent (B/C) fibers. Thebicomponent fibers comprise 60% PE and 40% PP. The downstream tier 38 isa high efficiency MB component with a weight of 5–100 g/m², preferably24 g/m². Notably, the independently composed tiers 37 and 38 meet atinterface 36A. This interface is different from that in a laminate oftwo pre-bonded layers in a multilayer composite. Due to the fact thatformation of a pre-bonded layer is not needed to produce the structure36, at least one of tiers 37 and 38 can be sufficiently flimsy that itcould not be formed into a free standing web to be incorporated as alayer in a conventional multilayer composite.

The upstream tier has an absolute pore volume of 21.4 cm³/g, thedownstream tier of 7.7 cm³/g, resulting in an ratio of absolute porevolume RAPV=2.78. The absolute projected fiber coverage, i.e. the unitarea which is covered by fibers when perpendicularly looking at thetier, of the upstream tier APFC is 97.7%. APFC of the downstream tier is99.3%.

To optimize the dust holding capacity, a ratio of average pore diameterof upstream tier to downstream tier of 6.21 is realized, the averagepore diameter of the upstream tier being 87 micrometer, the average poresize of the downstream tier being 14 micrometer.

In order to obtain the above RAPV and APFC values, the upstream tiercomprises a thickness of 1.7 mm, an apparent density of 0.044 g/cm³, anda relative pore volume of 94.4%. The downstream tier comprises athickness of 0.21 mm, an apparent density of 0.11 g/cm³, and a relativepore volume of 87.4%. It is understood that these values are exemplaryonly; in particular the above RAPV and APFC values can also be obtainedwith different thickness, apparent density, and relative pore volume.

FIG. 2 illustrates the highly improved pressure drop across the filterdepending on the amount of dust filtered by the composite filter. Theupper curve shows the composite filter with the characteristicsdiscussed above. The lower curve shows a prior art filter, consisting ofa spunbond as upstream tier and a meltblown as downstream tier. Theprior art upstream tier has an absolute pore volume of 6.9 cm³/g, thedownstream tier of 8.1 cm³/g, resulting in an ratio of absolute porevolume RAPV=0.85. The absolute projected fiber coverage of the upstreamtier is APFC 69.3%. APFC of the downstream tier is 92.3%.

A further embodiment (not shown) has the same structure as theembodiment shown in FIG. 1. This embodiment, however, comprises anupstream tier in form of a Dry-Laid FP Capacity tier with a weight of 50g/m². The FP layer has different blends of pulp fibers and bicomponent(B/C) fibers. The bicomponent fibers comprise 60% PE and 40% PP. Thedownstream tier is a high efficiency MB component with a weight of 24g/m². The upstream tier has an absolute pore volume of 22.7 cm³/g, thedownstream tier of 7.7 cm³/g, resulting in an ratio of absolute porevolume RAPV=2.95. The absolute projected fiber coverage of the upstreamtier is APFC 99.9%. APFC of the downstream tier is 99.3%.

To optimize the dust holding capacity, a ratio of average pore diameterof upstream tier to downstream tier of 5.93 is realized, the averagepore diameter of the upstream tier being 83 micrometer, the average poresize of the downstream tier being 14 micrometer.

In order to obtain the RAPV and APFC values, the upstream tier comprisesa thickness of 1.2 mm, an apparent density of 0.042 g/cm³, and arelative pore volume of 94.7%. The downstream tier comprises a thicknessof 0.21 mm, an apparent density of 0.11 g/cm³, and a relative porevolume of 87.4%.

In a further embodiment (not shown) the upstream tier comprises splitfilm fibers and “mixed electrostatic fibers.” Split film fibers and“mixed electrostatic fibers” are not used in all variations of theupstream tier, but at least 10% and preferably at least 20% B/C fibersor other types of thermally bondable fusing fibers should be used toachieve adequate thermal bonding. Generally, at least 10% and preferablyat least 20% pulp fibers are used for enhanced cover and filtrationefficiency. The tier can be free of B/C fibers or other types ofthermally bondable fusing fibers if latex binder is used.

FIG. 3 depicts a unitary composite filter 39 composed of three tiers.The first tier 40 is a coarse drylaid component made of 100% B/C fibers.It mainly serves as a pre-filter and protects downstream filtermaterial. The broadest weight range is 10–100 g/m² with a typical weightrange of 20–80 g/m², and a preferred weight of 50 g/m². The upstreamtier 41 is a Dry-Laid FP Capacity component as discussed in the aboveembodiments. The downstream tier 42 consists of high filtrationefficiency MB media or other ultrafine fiber diameter materials such asSpunBlown Modular or Microdenier Spunbond.

FIG. 4 is a diagram of a unitary composite filter 43 made from fourtiers of material. The first tier 44 is composed of Dry-Laid FP of 100%B/C fibers. The broadest weight range is from 10–100 g/m², typicalweight is from 20–80 g/m² and the target weight is 50 g/m². The upstreamtier 45 is a Dry-Laid FP Capacity tier as discussed in the aboveembodiments. Alternatively, tier 45 may contain at least 10% andpreferably at least 20% B/C fibers, 10% and preferably at least 20% pulpfibers, and may contain varying amounts of charged or uncharged splitfilm fibers. It may contain varying amounts of “mixed electrostaticfibers”. At least 10% and preferably at least 20% BIC fibers or othertypes of thermally bondable fusing fibers should be used to achieveadequate thermal bonding. Generally, at least 10% and preferably atleast 20% pulp fibers are used for enhanced cover and filtrationefficiency. The tier can be free of B/C fibers or other types ofthermally bondable fusing fibers if latex binder is used. The downstreamtier 46 contains MB filter media as discussed with respect to the aboveembodiments. The outer tier 47 is a Dry-Laid FP composed of air-laidpulp and B/C fibers.

FIG. 5 is a diagram of a unitary composite filter 48 made from fivetiers of material. The first tier 49 is composed of Dry-Laid FP of 100%B/C fibers. The broadest weight range is from 10–100 g/m², typicalweight is from 20–80 g/m² and the target weight is 50 g/m². The upstreamtier 50 is a Dry-Laid FP Capacity component as discussed above.Component 51 contains carbon granules or carbon fibers to absorb odorsand to remove pollutant and toxic gases from the air. Component 52 is ahigh filtration efficiency MB as discussed with respect to the aboveembodiments. Component 53 is a Dry-Laid FP composed of air-laid pulp andB/C fibers.

FIG. 6 depicts a unitary composite filter 54 of the same construction asshown in FIG. 1, composed of two tiers 55, 56, bonded to a supportingouter layer 57 consisting of a paper, scrim or nonwoven with a weightranging from 10–100 g/m².

FIG. 7 depicts a unitary composite filter 58 of the same construction asshown in FIG. 3, composed of three tiers 59, 60 and 61, bonded to anouter layer 62 consisting of a paper, scrim or nonwoven with a weightranging from 10–100 g/m².

FIG. 8 depicts a unitary composite filter 63 of the same construction asFIG. 4, composed of four tiers 64–67, bonded to an outer layer 68consisting of a paper, scrim or nonwoven with a weight of 10–100 g/m².

FIG. 9 depicts a unitary composite filter 69 of the same construction asFIG. 5, composed of five tiers 71–75, bonded to an outer layer 76consisting of a paper, scrim or nonwoven with a weight of 10–100 g/m².

FIG. 10 depicts a laminate of unitary composite filter 77 of the sameconstruction as shown in FIG. 1, composed of two tiers 78, 79, bonded toa supporting outer layer 81 consisting of a paper, scrim or nonwovenwith a weight ranging from 10–100 g/m², wherein the outer layer isbonded by glue or an adhesive 80, in which the latter could be a latexbinder or a hot melt adhesive.

FIG. 11 depicts a laminate of unitary composite filter 82 of the sameconstruction as shown in FIG. 3, composed of three tiers 83–85, to anouter layer 87 consisting of a paper, scrim or nonwoven with a weightranging from 10–100 g/m², wherein the outer layer is bonded by glue oran adhesive 86.

FIG. 12 depicts a laminate of unitary composite filter 87A of the sameconstruction as FIG. 4, composed of four tiers 88–91, to an outer layer93 consisting of a paper, scrim or nonwoven with a weight of 10–100g1mA², wherein the outer layer is bonded by glue or an adhesive 92.

FIG. 13 depicts a laminate of unitary composite filter 94 of the sameconstruction as FIG. 5, composed of five tiers 95–99, to an outer layer101 consisting of a paper, scrim, or nonwoven with a weight of 10–100g/mA², wherein the outer layer is bonded by glue or an adhesive 100.

Where bonding between layers is indicated in embodiments of FIGS. 10–13,conventional interlayer bonding methods, such as ultrasonic bonding canbe used in place of or in conjunction with glue/adhesive bondingmentioned above.

A preferred process for producing an embodiment of the novel compositefilter comprising a unitary stratified structure of MB and FPcompositions is shown in FIG. 14. The illustrated process provides aproduct laminated to a scrim, paper or nonwoven to facilitate handling,pleating or packaging. It is also possible to provide an unlaminatedcomposite filter by replacing the scrim, paper or nonwoven with asupporting conveyor to carry the non-prebonded tiers through theprocess. The final unitary composite filter consists of at leasttwo-tiers, although each tier may contain more than one type of fiber orother materials as discussed above, and generally consists of three tofive tiers, which are thermally or latex bonded. The electrostaticcharging of the composite filter is preferably done in-line by theTantret “cold” electrostatic charging process, although MB fibers may be“hot” charged in-line upon exiting the MB die. Also, split film fibers,which were electrostatically charged during their production, may beintroduced by the FP applicators. Furthermore, “mixed electrostaticfibers” which have opposite polarities after rubbing against each otherdue to different triboelectric properties may be incorporated into thecomposite by the FP applicators.

Now referring to FIG. 14, an optional unwind I is located at thestarting end of the line to allow for the feeding in of an optionalsupport layer 2, which may be a scrim, paper or nonwoven. Components 1,2, 4 and 5 are optional in that the inventive unitary composite filteris laminated to a scrim, paper or nonwoven only to facilitate handling,pleating or packaging. A conveyor belt 3 runs the entire length of theline; however, it may also be separated into shorter sections with oneconveyer section feeding the assembly of tiers onto the next sections asrequired in the process. Also at the starting end of the line there isan optional adhesive applicator 4 for dispensing an adhesive 5 in theform of glue or hot melt adhesive. This adhesive application station canbe utilized when it is desired to in-line laminate a supporting layer tothe unitary stratified structure of the novel composite. However, itshould be noted that applicator 4 is not intended for pre-bonding tierswithin the stratified structure.

Next, as shown in FIG. 14, there are at least one, and preferably two,FP applicator units 6 and 8. The primary function of the FP applicatorunits at the beginning of the line is to produce and deposit dry-laidtiers 7 and 9 onto the optional adhesive tier 5, or onto the conveyorbelt 3 if the optional support layer 2 and adhesive 5 are not used. Thedry-laid tiers 7 and 9 may be the same or have different compositionsand properties to meet the requirements of the end product. In anyrespect, the role of tiers 7 and 9 is primarily to support and protectthe MB or related filter media tiers 12 and 14. In the illustratedembodiment, the FP tiers 7 and 9 are primarily composed of “pulp” andbicomponent (B/C) fibers. Different types of B/C fibers may be used asdescribed above. For example, a preferred type has a core of highermelting point fibir such as PP and a sheath of lower melting point fibersuch as PE. Other preferred compositions of “pulp” and B/C core sheathPP/PE are 50% “pulp”/50% B/C fibers in tier 7 and 25% “pulp” 75% B/Cfibers in tier 9. If latex binder is not applied in section 23, then atleast 20% B/C fibers or other types of thermal binder fibers should beused. On the other hand, if latex bind is subsequently applied insections 23 and 27, then 100% “pulp” fiber can be applied by FPapplicators heads 6 and 8. Also, it is possible to apply 100% B/C fibersfrom FP applicator 6 or applicator 8, or from both applicator heads 6and 8.

In additional embodiments, instead of 100% B/C fibers, monocomponentregular staple fibers of PP, PET, polyamide and other fibers can besubstituted for up to 80% of the B/C or thermal bonding fibers that maybe applied by any of the FP application heads 6, 8, 15, 18, and 20. Manytypes of thermally bondable fusing fibers which completely melt and arealso known as “melt fibers” also can be used in place of the B/C fibers,except in dry-laid tier components where 100% B/C fiber would be used.

FIG. 14 further illustrates optional compactor 10 which decreases thethickness of the web and increases fiber-to-fiber adhesion of FP tiers 7and 9. It should be noted that the extensive pre-bonding typicallyemployed to separately produce the layers is not the objective of thisoptional compacting step utilized in this inventive in-line process. Thecompactor 10 may be a calender, which may or may not be heated. The MBor related filter media 12 and 14 may be deposited by one or more MBdies 11 and 13 onto the FP tiers 7 and 9. The primary function of the MBcomponent is to serve as a high efficiency filter, that is, to removesmall percentages of small size (less than about 5 micrometers)particles. The specifications of filtration grade MB media and relatedultrafine fiber diameter types of filter, media are given in Table I.

The process can include at least one or more MB dies 11 and/or one ormore related fine denier, (ultrafine fiber diameter) fiber applicators13, designated as X. For example, if two identical MB units areutilized, then units 11 and 13 will be the same. Other variationscontemplated to come within the breadth of this invention include havingthe first unit as a SpunBlown (Modular) or Microdenier Spunbond (SB)system first to form a filter gradient of coarser to finer highefficiency filters. Another contemplated variation is for one or moreSpunBlown (Modular) or Microdenier SB systems to be used in tandem.Still another variation is to use a Microdenier SB first followed by aSpunBlown system.

The next equipment component shown in FIG. 14 is another FP applicator15, which deposits an FP web on top of tier 14 (or on tier 12 if asecond MB tier 13 is not included). Then the non-prebonded assembly oftiers with tier 16 uppermost travels through another optional compactor17. Next the intermediate product is conveyed beneath one or moreadditional FP units 18 and 20. FP applicator heads 15 and 18 incorporatethe Dry-Laid Capacity tier into the structure. FP applicator 20 isprimarily designed to produce very open (i.e. bulky) FP primarily fordust holding capacity rather than as a filter. The very open FP tier 21preferably is produced from 100% bicomponent B/C fiber or blends of B/Cwith ratios of B/C to “pulp” characterized as being higher than isnormally used to produce coarse pre-filter FP webs. Either or both FPtiers 16 and 19 can also contain split film fibers and “mixedelectrostatic fibers”. If no B/C fibers or other types of thermalbonding fibers are used in FP tiers 16 and 19, then latex binder shouldbe applied at units 23 and 27 to bond the tiers. If B/C fibers or othertypes of thermal bonding fibers are incorporated in either of FPapplicator heads 15 and 18, then latex binder still can also be appliedat units 23 and 27.

The intermediate product with uppermost tier 21 then travels throughanother compactor 22 and thence through a section of the production linewhere the previously loose, unbound tiers are subjected to one or morebinding process steps that are cumulatively effective to form theunitary stratified structure of the composite filter. Preferably, all ofthe filter components that will be incorporated into the unitarystratified structure are incorporated in the intermediate product atthis stage prior to binding the tiers together.

With further reference to FIG. 14, it is seen that the binding stepstake place beginning in, the illustrated embodiment with a latex binder24 being applied by applicator 23. The latex can be sprayed from aliquid dispersion or emulsion, applied by kiss roll or gravureapplication, or sprayed as a dry powder onto the substrate and thenthermally fused or bonded thereto. The latex also serves as a sealant inthat it minimizes dust that can emanate from outside surfaces of the FPtier. After adding latex binder at 23, the intermediate product travelsthrough a heating unit 25 which dries and cures the latex binder to bondthe composite. The heating unit can be a heated calender, or aninfrared, microwave, or convection oven. A combination of these can alsobe used. A through-air oven is preferred. If B/C fibers or other typesof thermally bondable fusing fibers are present in the intermediateproduct, then ovens 25 and 29 can serve to thermally fuse such fibers tocontinue the bonding and formation of the unitary structure.

From the oven 25, the intermediate product is cooled by system 26, andthen a second latex binder is applied at 27. As illustrated, the path oftravel and spraying unit 27 are positioned to apply latex binder to theside opposite the first application. The intermediate product containingthe second latex binder 28 then passes through a second through-air oven29 and through another cooling section 30. Next, the fully bondedcomposite film having a unitary stratified structure is charge in coldelectrostatic charging station 31, preferably, a Tantret J system.Finally, the composite film 32 is slit to desired width or multiple ofwidths on shiter 33 and rolled up by the winder 34. Althoughelectrostatic charging is illustrated to take place toward the end ofthe process, it is contemplated that charging at a stage prior t6application of latex binder can be performed, provided that the binderand the subsequent procedural steps do not significantly drain thecharge from the intermediate product.

1. A composite filter for filtering a stream of ambient air comprisingat least one non-prebonded upstream tier and one non-prebondeddownstream tier, wherein the ratio of absolute pore volume of upstreamtier to downstream tier RAPV>2, and the absolute projected fibercoverage of upstream tier and of downstream tier APFC>95%.
 2. Thecomposite filter of claim 1, wherein the ratio of average pore diameterof upstream to downstream tier RPD is 4<RPD<10.
 3. The composite filterof claim 2, wherein the average pore diameter of the upstream tierPDU>60 μm.
 4. The composite filter of claim 1, wherein the upstream tiercomprises a relative pore volume RPVU>94%, an apparent density ADU<0.05g/cm³, and a thickness D in the range of 0.5 mm<D<2.5 mm.
 5. Thecomposite filter of claim 4, wherein the downstream tier comprises arelative pore volume RPVD being smaller than RPVU, an apparent densityADD in the range of 0.07 g/cm³<ADD<0.14 g/cm³, and a thickness D in therange of 0.1 mm<D<0.4 mm.
 6. The composite filter of claim 1, whereinthe upstream tier comprises fibers having a length in the range of 0.1mm to 3.0 mm.
 7. The composite filter of claim 6, wherein theorientation of the fibers in flow direction in the upstream tier ishigher than in the downstream tier.
 8. The composite filter of claim 1,wherein the upstream tier comprises a dust retention DR with respect todust particles with a diameter corresponding to the average porediameter of the downstream tier of DR>99%.
 9. The composite filter ofclaim 1, wherein the upstream tier comprises dry-laid, thermallybondable fusing, bicomponent or monocomponent polymer fibers and thedownstream tier comprises meltblown fibers.
 10. The composite filter ofclaim 9, wherein the upstream tier has a composition selected from thegroup consisting of 100 wt % bicomponent polymer fibers, a blend of atleast about 10 wt % bicomponent polymer fibers with a complementaryamount of natural fibers, staple fibers or a mixture thereof, and ablend of at least about 10 wt % monocomponent polymer thermally bondablefusing fibers with a complementary amount of fluff pulp fibers, staplefibers or a mixture thereof.
 11. The composite filter of claim 10,wherein the bicomponent polymer fibers have a sheath of one polymer anda core of a different polymer having a melting point higher than the onepolymer.
 12. The composite filter of claim 11, wherein the core ispolypropylene and the sheath is polyethylene.
 13. The composite filterof claim 12, wherein the core is disposed eccentric relative to thesheath.
 14. The composite filter of claim 9, wherein the upstream tierfurther comprises fibers selected from at least one of uncharged splitfilm fibers, charged split film fibers and mixed electrostatic fibers.15. A vacuum cleaner bag comprising a composite filter in accordancewith claim
 1. 16. A method of making a composite filter in accordancewith claim 1, comprising the steps of (a) laying down a filtrationmaterial onto a support to form the upstream non-prebonded tier, (b)depositing onto the upstream tier the downstream non-prebonded tier, and(c) bonding the tiers to form a composite filter having a unitarystratified structure.
 17. A composite filter produced by the method ofclaim
 16. 18. A vacuum cleaner bag comprising a composite filterproduced by the method of claim
 16. 19. The composite filter of claim 2,wherein the average pore diameter of the upstream tier PDU is >80 μm and>200 μm.
 20. The composite filter of claim 1, wherein the upstream tiercomprises a relative pore volume RPVU>96%.